The first takeaway my friend and I had reading this is that the 'Adventurer/Hybrid' class allows for some really fantastic and cool high-concept characters. I think its my favourite of the additions, followed closely by the new combat system.

If I may though - there's a small typo on the Skill List. Final sentence of 'Connect' has "You can use it it place..."

Artificer Armour refers to bespoke high-grade Power Armour. There are non-Astartes Power armour variants employed by the Adepta Sororitas and also by Inquisitors that are less efficient and powerful due to the lack of implants and Space Marine physiology that empowers typical Power Armour. It will largely have the same protection capabilities but will not enhance the user anywhere near Astartes Armour will.

Its difficult to explain, so bare with me. The Elven spirit is a distinct entity and the more 'truer' self of the Elves. Their body is just the meatbag that the spirit exists in and consumes. The fading and passing of the Elves in Middle-Earth is a consequence of the potency of the Elven spirit, it burns their meatsuits out eventually. This however, doesn't occur in the West.

If the body dies, the Spirit must return to the West to reincarnate back into a new body. This isn't a function or a power of the Elves themselves, its done and granted by the Valar themselves. Mandos was chiefly responsible for this. If the Elven spirit didn't to return to him, it would thusly remain as a Spirit, he would also deny them reincarnation as well if given cause.

This is a simplified and broad strokes answer as I understand them, don't take it as absolute truth.

Luthien is a different exception given the manner in which she returned. Glorfindel returned wholesale and intact.

There are various references in Tolkien's letters that suggest an Elven spirit can remain in Middle Earth, if they refuse to return to the West or are unable due to great taint they would remain. Strictly speaking they should by rights by completely invisible, so Celembrimbor being a wraith isn't wholly implausible.

Generally speaking - I improvise at least 75% of anything I do, this of course works better some times than others. The best advice I can give you is find the ratio and style that suits you - I initially got introduced to being a GM from very heavy planners, I tried to emulate and failed horribly. I was later exposed to a more free-flow style that suited me quite well and enabled me to really breakthrough the rut I was stuck in.

My process is to generally decide the overall synopsis of an adventure I'll run, no more than a paragraph of the plot. I'll think of any dramatis personae that are essential, everything else I'll make up on the spot or as the need requires.